1. Field of the Invention
The present invention relates to an antenna with which a digital signal or an analog high-frequency signal, e.g., that of a microwave range or an extremely high frequency range, is transmitted or received.
2. Description of the Related Art
For two reasons, wireless devices are desired which are capable of operating in a much wider band than conventionally. A first reason is the need for supporting short-range wireless communication systems, for which the authorities have given permission to use a wide frequency band. A second reason is the need for a single terminal device that is capable of supporting a plurality of communication systems which use different frequencies.
For example, a frequency band from 3.1 GHz to 10.6 GHz, which has been allocated by the authorities to short-range fast communication systems, corresponds to a bandwidth ratio as wide as 109.5%. As used herein, “a bandwidth ratio” is a bandwidth, normalized by the center frequency f0, of a band. On the other hand, patch antennas (known as a basic antenna structure) have bandwidth ratio characteristics of less than 5%, whereas slot antennas have bandwidth ratio characteristics of less than 10%. With such antennas, it is very difficult cover the entirety of the aforementioned wide frequency band.
In a preliminary version of specifications which are contemplated for the aforementioned communication systems, it is assumed that the authorized frequency band is to be used while being divided into a plurality of portions. One reason thereof is the difficulty to realize an antenna which covers the entirety of an ultrawideband (UWB) with the currently-available technology.
To take for example the frequency bands which are currently used for wireless communications around the world, a bandwidth ratio of about 30% must be realized in order to cover from the 1.8 GHz band to the 2.4 GHz band with the same antenna. In order to cover also the 800 MHz band and the 2 GHz band in addition to the aforementioned band with the same antenna, a bandwidth ratio of about 90% must be realized. Furthermore, in order to cover from the 800 MHz band to the 2.4 GHz band with the same-antenna, a bandwidth ratio of 100% or must be realized. Thus, as the number of systems to be supported by the same terminal device increases, and as the frequency band to be covered becomes wider, the need will increase for a wideband antenna, this being a solution for realizing a simple terminal device structure.
The ¼ wavelength slot antenna, whose schematic diagram is shown in FIG. 23, is one of the most basic planar antenna structures. FIG. 23A is an upper schematic see-through view; FIG. 23B is a schematic cross-sectional view taken along line AB; and FIG. 23C is a schematic see-through rear view, as seen through the upper face side.
The illustrated slot antenna has a feed line 261 provided on the upper face of a dielectric substrate 101. A recess 14 is formed which extends in the inward direction from an edge 12a of a finite ground conductor 12, which in itself is provided on the rear face. Thus, the recess 14 functions as a slot 14 having an open end 13. The slot 14 is a circuit element which is obtained by removing the conductor completely across the thickness direction in a partial region of the ground conductor 12. The slot 14 resonates near a frequency such that its slot length Ls corresponds to a ¼ effective wavelength.
The feed line 261, which partly opposes the slot 14, excites the slot 14. The feed line 261 is connected to an external circuit via an input terminal 201. Note that, in order to establish input matching, a distance t3 from a leading open-end point 20 of the feed line 261 to the center of the slot 14 is typically set to about a ¼ effective wavelength at the frequency f0.
Japanese Laid-Open Patent Publication No. 2004-336328 discloses a structure for operating a ¼ wavelength slot antenna at a plurality of resonant frequencies. FIG. 24A shows a schematic structural diagram thereof. In FIGS. 24A and 24B, those elements which have their counterparts in the antenna of FIG. 23 are denoted by the same reference numerals as their respective counterparts.
In the slot antenna of FIG. 24A, the ¼ wavelength slot 14 is excited at a feed point 15, whereby a usual antenna operation occurs. The resonant frequency of a slot antenna is usually defined by the loop length of the slot 14. In the illustrated antenna, a capacitor element 16 which is provided between a point 16a and a point 16b is prescribed so as to allow a signal at any frequency that is higher than the intended resonant frequency of the slot 14 to pass through. This makes it possible to vary the resonator length of the slot 14 depending on frequency. Specifically, at lower frequencies, as shown in FIG. 24B, the resonator length of the slot 14 does not change from its usual value, and therefore is determined by the physical length of the recess structure. At higher frequencies, on the other hand, the antenna operates as if the resonator length of the slot 14 were shorter than the actual, physical resonator length, as shown in FIG. 24C. Japanese Laid-Open Patent Publication No. 2004-336328 describes that, based on the above construction, a single slot structure can attain a multiple resonance operation.
Japanese Laid-Open Patent Publication No. 2004-23507 discloses a structure for allowing a ½ wavelength slot antenna to resonate at a plurality of frequencies. FIG. 25 is a see-through view as seen from the side of a rear face ground conductor. As shown in this figure, in Japanese Laid-Open Patent Publication No. 2004-23507, a plurality of slots 14a, 14b and 14c, which are of sizes respectively satisfying the resonance condition for a plurality of desired frequencies, are provided within the structure of a ground conductor 12. Then, the slots 14a, 14b and 14c are excited at points 51a, 51b and 51c, where a ¼ effective wavelength is obtained for each frequency (beginning from an open-end 20 of a feed line 261), whereby multiple resonance is realized. Note that a pattern which is shown by a solid line in FIG. 26 indicates a conductor pattern on the rear face of the substrate, whereas a pattern shown by a dotted line indicates a conductor pattern on the front face of the substrate.
“A Novel Broadband Microstrip-Fed Wide Slot Antenna With Double Rejection Zeros” IEEE Antennas and Wireless Propagation Letters, vol. 2, 2003, pp. 194 to 196 (hereinafter “Non-Patent Document 1”) discloses another method for realizing a wideband operation of a ½ wavelength slot antenna. As mentioned above, one input matching method for a conventional slot antenna has been to excite the slot resonator 14 at a point where a ¼ effective wavelength at the frequency f0 is obtained, beginning from the leading open-end point 20 of the feed line 261. However, in Non-Patent Document 1, as shown in FIG. 26 (which shows an upper schematic see-through view), a region spanning a distance corresponding to a ¼ effective wavelength at the frequency f0, beginning from a leading open-end point 20 of a feed line 261, has a narrower line width so as to form a high-impedance region 263. The transmission line in the high-impedance region 263 has a higher characteristic impedance than the characteristic impedance (50Ω) of the normal transmission line, and is coupled to a slot 14 in an approximate center thereof.
In terms of equivalent circuitry, the newly-introduced high-impedance region 263 functions as a resonator which is different from the slot resonator. According to Non-Patent Document 1, such a construction increases the number of resonators to two, and a multiple resonance operation can be obtained by coupling the two resonators. FIG. 2B of Non-Patent Document 1 shows the frequency dependence of return intensity characteristics obtained under the conditions described in Table 1 below.
TABLE 1dielectric constant of substrate2.94substrate thickness0.75mmslot length (Ls)24mmdesign frequency5GHzt1 + t2 + Ws9.8mmline width W20.5mmoffset distance from feed line 261 to slot center9.8 mm to 10.2 mm
According to Non-Patent Document 1, in the above-described range of offset distance, return intensity characteristics as good as −10 dB or less are obtained with a bandwidth ratio 32% (from near 4.1 GHz to near 5.7 GHz). Such band characteristics are much better than the bandwidth ratio of 9% of a usual slot antenna which is produced under the same substrate conditions, as shown in comparison with measured characteristics that are illustrated in FIG. 4 of Non-Patent Document 1.
The aforementioned conventional slot antenna has a problem in terms of wideband-ness.
Firstly, the operating band of a usual slot antenna, which only has a single resonator structure within its structure, is restricted by the band of its resonance phenomenon. As a result of this, the frequency band in which good return intensity characteristics can be obtained only amounts to a bandwidth ratio of less than about 10%.
Although the antenna of Japanese Laid-Open Patent Publication No. 2004-336328 realizes a wideband operation because of a capacitive reactance element being introduced in the slot, there is a problem in that an additional part such as a chip capacitor is required as the actual capacitive reactance element. There is also a problem in that variations in the characteristics of the newly-introduced additional part may cause the antenna characteristics to vary. Furthermore, according to the example disclosed in Japanese Laid-Open Patent Publication No. 2004-336328, there is also a problem associated with the band characteristics. For example, FIG. 14 of Japanese Laid-Open Patent Publication No. 2004-336328 shows an example indicating a multiple resonance operation at 1.18 GHz and 2.05 GHz, but at each frequency, there is only about several tens of MHz of a band in which the VSWR (Voltage Standing Wave Ratio) is less than two. FIG. 18 of Japanese Laid-Open Patent Publication No. 2004-336328 shows an example where a VSWR of less than three is being obtained in a band from 1.7 GHz to 3.45 GHz, which would correspond to a bandwidth ratio of 66%. However, such a band is still insufficient, and a VSWR of about three cannot be considered as representing good return intensity characteristics.
Thus, according to the disclosure of Japanese Laid-Open Patent Publication No. 2004-336328, it is difficult to provide an antenna which attains low-return input matching characteristics in a ultrawide frequency band that is currently desired.
The method of Japanese Laid-Open Patent Publication No. 2004-23507 will prove extremely difficult in practice. Specifically, since the feed line 261 intersects a number of slots between the input terminal and the leading open-end point, a considerable impedance mismatch is predicted. It is even possible that, in each frequency band where the resonant bands of the respective slots overlap one another, good antenna operation may be hindered by a coupling between the adjoining slots. In the case where the plurality of slots introduced in the structure do not have any overlaps between their resonant bands, impedance matching could be realized in each separate frequency band. However, since each slot has a 10% band in actuality, and a different mode of antenna operation will occur also in each spurious band (e.g., second harmonic and third harmonic), there will only be a very limited frequency band in which the desired return intensity characteristics and radiation characteristics are reconciled. In either case, it will be difficult for this structure to achieve a bandwidth ratio of several tens of % or more.
Also in the example of Non-Patent Document 1, where a plurality of resonators are introduced in the structure in order to improve the band characteristics based on coupling between the resonators, the bandwidth ratio characteristics are only as good as about 35%, which needs further improvement. The upper schematic see-through view of FIG. 26 (which is modeled after FIG. 1 of Non-Patent Document 1) illustrates the slot width Ws to be of a small dimension. However, under the conditions for obtaining the aforementioned wideband characteristics, the slot width Ws will have to be set to 5 mm, which accounts for more than half of the length of ¼ wavelength region, i.e., 9.8 mm. When a desire for downsizing the antenna permits only a limited area for accommodating the slot, it may become necessary to fold up the linear-shaped slot, for example. Thus, a structure which requires a large Ws value in order to obtain wideband characteristics will be difficult to be downsized by nature.